Amid a strong need for alternative energy such as a clean energy in efforts to prevent global warming, the development of technology for converting sunlight: into a secondary energy (electric power, hydrogen, etc.) in an efficient manner is urgently needed. Expectations are rising for systems such as photovoltaics and hydrogen generating photocatalysts, etc., having high light-conversion efficiency. In the solar cells or the hydrogen generating photocatalysts etc., only the wavelength regions shorter than a specific threshold wavelength that is inherent to the system have been used among the broad wavelength range of sunlight. Thus, as one of the technologies for effectively utilizing the broad wavelength range of the sunlight, photon upconversion (in which light of a long wavelength is absorbed and then light of a shorter wavelength is emitted, thereby changing the wavelength of the light.) is being investigated.
Research on photon upconversion using multiphoton absorption of rare-earth elements as a means for photon upconversion has a history of 50 years or more. However, multiphoton absorption of rare-earth elements generally requires a very high incident light intensity, which has made this method difficult to be applied to weak light such as sunlight as a target for conversion.
In recent years, organic molecules capable of conducting photon upconversion by light absorption and emission were disclosed.
Patent document 1 describes a composition for upconverting a photon energy, the composition comprising a first component that absorbs energy at a first wavelength region by serving as a photoreceptor of at least phthalocyanine, metal porphyrin, metal phthalocyanine etc., and a second component that emits energy at a second wavelength region by serving as a light emitter of polyfluorene, oligofluorene, copolymer thereof, poly-paraphenylene, etc.
Non-patent document 1 describes photon upconverters that use a triplet-triplet annihilation process (hereinafter referred to as “TTA process”) between organic molecules, which up-converted relatively weak light whose intensity is close to sunlight in toluene solvent.
As a medium for organic molecules in a photon upconverter, there is a precedent example that used a high molecular weight organic polymer.
Patent document 2 describes the use of a system for photon upconversion comprising at least one polymer and at least one sensitizer containing at least one heavy atom, wherein the triplet energy level of the sensitizer is higher than the triplet energy level of the polymer.
Non-patent document 2 describes a photon upconverter wherein cellulose acetate polymer (molecular weight about 100,000) was used as a dispersion medium for organic molecules. However, the Non-patent document 2 discloses no quantitative data on the upconversion quantum yield of the photon conversion element.
Non-patent document 3 describes a photon upconverter which uses, as a medium, rubbery polymer that has a glass transition temperature (Tg) of 236K (−37° C.) and which is soft at room temperature. The Non-patent document 3 describes that, because the photon upconversion based on the TTA process requires exchange of energies between organic molecules carrying a triplet excitation energy through their diffusive motions and resultant intermolecular collisions within the medium, the intensity of the upconversion light emission increases in a relatively high temperature range (>300K) in which the polymer has sufficient fluidity, but in the lower temperature range (≦300K) in which the fluidity of the medium is low, the intensity of the upconversion light emission becomes very weak. However, the Non-patent document 3 discloses no quantitative data on the upconversion quantum yield of the photon upconverter.
Non-patent document 4 describes photon upconverters that uses styrene oligomer (a mixture of styrene trimers and tetramers) as a medium for organic photosensitizing molecules and organic light-emitting molecules. The Non-patent document 4 describes that, when a sample was scanned at about 10 kHz with a laser having an output of about 14 W/cm2 as an excitation light, the upconversion quantum yield of as high as 3.2% was obtained. While the Non-patent document 4 uses its unique index called “the mean excitation intensity” in order to represent a light excitation intensity and describes that the index was about 5 mW/cm2, it does not clearly specify the definition of this unique index.
Non-patent document 5 describes that metal porphyrins and metal phthalocyanines are usable as organic photosensitizing molecules, and molecules such as 9,10-bis(phenylethynyl)anthracene, perylene, rubrene etc., are usable as light-emitting molecules, for photon upconversion that uses the TTA process.
Non-patent document 6 describes a general review on ionic liquids, stating that the nature of an ionic liquid is, as shown in FIG. 1 cited from the Non-patent document 6, usually nonflammable, has negligible vapor pressure under normal conditions, and it is questionable whether the concept of polarity and non-polarity may directly apply to the ionic liquid or not, and the like.
Non-patent document 7 describes, based on experimental results, the miscible proportions of a non-water miscible ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) with several general organic solvents. The Non-patent document 7 describes that the proportion (miscible proportion) at which an organic solvent can mix homogeneously with an ionic liquid without layer separation depends on either or both of the polarity (dipole moment: D) and size of an organic solvent molecule, and that the higher the polarity of the organic solvent molecule becomes, the possible mixing ratio of the organic solvent in the non-water miscible ionic liquid increases.
Non-patent document 8 describes that experiments of measuring the polarity of various ionic liquids that have 1-alkyl-3-methylimidazolium as a cation had been carried out, and that, based on the result of the experiments, these ionic liquids have polarities comparable to those of short chain alcohols.
Non-patent document 9 describes that upon trying to determine the properties of porphyrins, which are polycyclic aromatic π electron-conjugated molecules, in an ionic liquid, the obtained signal was very weak because the molecules hardly dissolved in the ionic liquid. Regarding this fact, the authors of this article stated “The polarity of room-temperature ionic liquids are similar to acetonitrile and alcohols such as methanol and 2-propanol according to the previous studies, and we assume that typical aromatic compounds are not soluble in room-temperature ionic liquids.”